Display media such as plasma panels, liquid crystal displays as well as ink on paper have been used to display continuous tone pictures using a variety of techniques which transform a multilevel signal representing the intensity values of an original picture into a pseudo-gray scale signal in which only a small number of levels are used to represent the intensity of each picture element (pel). In general, these techniques take advantage of the spatial low-pass filtering that occurs in the human visual system to achieve an illusion of a gray-scale rendition, even though each cell or display site within the display device is capable of residing only in a limited number of states. This is done by controlling the proportion or density of display cells in a given neighborhood that are in each state, in accordance with the intensity values of picture elements (pels) in the corresponding region in the picture. U.S. Pat. No. 3,997,719 issued to C. N. Judice describes one specific pseudo-gray scale display technique which has found acceptance, particularly in connection with plasma panel displays, in which each display cell is capable of residing in one of two possible states, namely, fully "on" (energized) or fully "off" (deenergized). With this technique, called "ordered dither", a series of multilevel samples which represent the intensity values of elements in the picture to be displayed are compared to a series of position dependent threshold values which form a dither matrix, to generate a one-bit "display word" for each picture element. Each spatially corresponding cell in the bilevel display is then energized in accordance with its appropriate display word. For example, if the one-bit display word is a "one", this is taken to mean that the intensity exceeds the threshold value, and the cell is turned "on". Conversely, a display word of "zero" indicates that the intensity falls below the threshold, and the cell is maintained in an "off" state. The same dither matrix is generally repeated numerous times throughout the entire display area. Each individual matrix includes a plurality of different threshold values which lie within a range determined by the maximum and minimum intensity values which can be assumed by the input signal. Advantageously, the threshold values are distributed within each dither matrix so as to achieve maximum spacing between numerically successive values.
While the n-bit display words which form the pseudo-gray scale signal are primarily intended for use with a display device in which each display site can exhibit only a limited number of intensity values, it is sometimes desirable to utilize a multilevel or continuous display medium such as a CRT for display purposes. In this event, the most straightforward approach is to convert each n-bit display word to one of 2.sup.n arbitrarily chosen intensity values which lie within the range of values which can be displayed, depending solely upon the value of the n-bit word. This approach, of course, does not utilize the full range of intensities that may be displayed by the CRT, nor does it take into account information which may be obtained from knowledge of the threshold value used to form the display word. It also often leads to undesirable instability or flicker, particularly where the CRT uses an interlaced-field format. In this format, each frame is formed from two fields each having half of the total number of lines desired. The lines from consecutive fields alternate, with one field including odd numbered lines and the next including even numbered lines. Flicker occurs because adjacent lines in a frame have different threshold values which often cause a noticeable difference in brightness.
One technique designed to avoid flicker when it is desired to display a pseudo-gray scale signal such as an ordered dither signal on a multilevel device is described in U.S. Pat. No. 3,953,668 issued to C. N. Judice. Here, the one-bit display words for a group of N picture elements in the neighborhood of the pel being processed are numerically averaged. Each of the N possible averages is assigned one of N different intensity values within the range of values capable of being displayed. Since this procedure tends to smooth or filter the bilevel signal, the flicker is removed. However, the resulting display is blurred, and information is lost in the averaging process. Also, the full capability of the multilevel display is not realized, because the number N of possible different average values is determined solely by the size of the neighborhood used for averaging.
In view of the foregoing, it is the broad object of the present invention to permit improved display of a picture signal which has been processed using a pseudo-gray scale algorithm on a device capable of displaying a large number of different levels. Specific objects are to fully utilize the gray scale capabilities of the display and to avoid blurring of the picture.